Electrode for plasma generator the generator comprising same and process for treatment of solidifying liquid metal

ABSTRACT

A main electrode ( 2, 20, 30, 44, 127 ) for plasma arc generator, a generator ( 50, 70, 80, 126 ) comprising same and a process for treatment of solidifying liquid metal by the mentioned generator, wherein the main electrode in association with a counter electrode ( 15, 28, 42, 54, 73, 86, 122 ) provides a two-rail structure capable of generating a plasma arc discharge displaceable along a closed path uninterruptedly. The uninterrupted movement of the arc discharge is achieved by a specific design of the main electrode. The electrode comprises an essentially tubular body having a first rim ( 3, 24, 33, 89 ) usually connected to a d.c. power source via at least one connector site ( 12 ), and a second, working rim ( 4, 27, 34, 46, 63, 78, 90 ) serving for the electric arc discharge. The tubular body is divided by at least one slot (gap) ( 6, 22, 32, 49, 52, 88 ) associated with one connector site and extending between the first and second rims so that it forms at the second rim region a second rim gap. Two sides of the second rim gap are an arc transmitting ( 16, 36 ) and an arc receiving ( 17, 35 ) zones, respectively. Mutual positions of these two zones and the associated connector site are such, that when the arc column is created and displaces along the second rim, it will always be transmitted from the transmitting zone to the receiving zone at a location positioned downstream from the projection of the associated connector site to the second rim (in respect of the direction of the plasma arc movement). Owing to this arrangement the arc column will cross the second rim gaps uninterruptedly.

FIELD OF THE INVENTION

The present invention relates to plasma arc generators of both thetransferable and non-transferable types, and more specifically to plasmaapparatus of the kind generating a plasma arc that circulates in aclosed path. The invention further relates to an electrode for use inplasma generators of the kind specified.

Plasma arc generators are used for the heat treatment of various objectsin numerous technological processes, for example in metallurgicalprocesses for so-called plasma remelting, plasma casting, plasmacleaning, etc. By one of its aspects, the invention relates to a processfor heating with a circulating plasma arc a liquid metal chilling andcrystallizing within a mold, with the object of eliminating typicalcasting defects, such as the formation of blowholes and porosity,segregation, formation of contraction cavities, inhomogeneity ofchemical composition and crystal structure across the ingot, etc.

BACKGROUND OF THE INVENTION

Plasma generators including plasma arc torches are known in the art, andgeneral descriptions of their design and of their use for variousmetallurgical applications, can be found in numerous technicalmonographs or handbooks, e.g. the chapter “Plasma Melting and Casting”in Metals Handbook, Ninth Edition, Vol. 15, Metals Park, Ohio, and themonograph “Plasma Metallurgy, The Principles” by V. Dembovsky, Elsevier,1985, p.314-315.

Basically, plasma generators can be divided into two groups: those inwhich both cathode and anode form part of the apparatus which are knownas plasma generators with non-transferable arcs or non-transferableplasma arc generators; and those which include only one electrode whilethe counter electrode is an electricity conducting substrate, which areknown as plasma generators with transferable arcs or transferable plasmaarc generators.

GB 1268843 describes a non-transferable plasma arc generator comprisinga water cooled cathode and two annular anodes, one for ignition and theother for regular operation, connected to a power supply. The cathodetip is protected by injection of an inert gas such as argon, helium ornitrogen.

U.S. Pat. No. 4,958,057 describes a typical transferable plasma arcgenerator for use to heat metal in a continuous casting process. Itcomprises a cylindrical cathode-holding member with water coolingarrangements, an ignition anode and a ring-shaped cathode, having aninner channel for the injection of an inert protecting gas. An electricdischarge is effected between the cathode and substrate to be treated,which is set as the anode.

It is an intrinsic disadvantage of the conventional plasma generators ofboth the non-transferable and transferable types, that for properfunctioning the injection of a protecting gas or water cooling arerequired. Where gas cooling is employed, so-called plasma torches areused which comprise a plasma delivery nozzle. Injection of a pressurizedinert gas into the torch is associated with the formation of anelongated plasma jet ejected at high velocity from the plasma deliverynozzle which in case of treatment of a solidifying cast metal causes theexertion of localized pressure on the surface of the still solidifyingmetal, leading to the formation of large cavities during chilling.

The presence of cooling water is dangerous because any leaking waterthat reaches the hot liquid metal may cause an explosion.

There are also known plasma generators in which a plasma arc iscontrollably displaced with respect to a treated substrate in an open,e.g. straight, or closed, e.g. circular fashion along a correspondinglyshaped electrode. Such displacement of the arc avoids overheating,provides for a more uniform treatment of the substrate and reduceserosion of the electrodes, thereby prolonging the life span of theapparatus. Thus U.S. Pat. No. 5,132,511 discloses a non-transferableplasma torch having two coaxial tubular electrodes axially spaced fromeach other and provided with an electromagnetic coil for rotating thearc. The coil is mounted in a sealed cylindrical chamber positionedbetween the two electrodes.

U.S. Pat. No. 5,393,954 describes a non-transferable plasma torch whichcomprises two coaxial tubular electrodes at least one of which issurrounded by a magnetic field associated with electronic control means,whereby the plasma arc foot is displaced in a controlled fashion. When aplasma-generating gas is injected into a chamber separating saidelectrodes, an arc is ignited.

It is known that the arc in a plasma generator may be displaced by theaction of a ponderomotive force known as the Lorentz force. A Lorentzforce arises when an electric charge moves in a magnetic field and isproportional to the magnetic induction of the field, the electriccharge, its velocity and also depends on the angle between the vectorsof magnetic induction and velocity of the moving charge. It is knownthat a Lorentz force is created in a plasma generator as a result ofinteraction between the arc (being an intensive electric discharge), itsmagnetic field, and the magnetic field created in the generator by theelectric current flowing through the electrodes. When the electrodesform a so-called two-rail structure the Lorentz force accelerates anddisplaces the electric arc.

The term “two-rail structure” used herein with reference to theelectrodes in plasma generators should be understood as meaning twoparallel current conducting objects (so-called rails) spaced from oneanother, and connected each to one of the electric power supply poles.When an electric arc is initiated between the electrodes, it moves alongthe rails away from the site of electric contact thereof with the powersupply.

In accordance with prior art terminology plasma arc generators in whichthe arc discharge is accelerated by a ponderomotive force within a spacebetween two parallel electrodes are sometimes referred to aselectromagnetic rail accelerators or plasma accelerators with railgeometry.

The phenomenon, by which the Lorentz force accelerates and displaces theplasma arc in a plasma arc generator with a two-rail structure, is knownas the principle of electromagnetic acceleration. It is mentioned in theliterature with reference to plasma accelerators or magnetichydrodynamic generators, e.g. in “Impulse Plasma Accelerators” byAlexandrov et al., Charkov, 1983, pp. 192, 194 and in “ElectroslagWelding and Melting” by J. Kompan and E. Sherbinin, Machinostroenie,1989, pp. 191, 192. A specific application of the Lorentz force isdescribed in “Scaling Laws for Plasma Armatures in Railguns” by LindseyD. Tornhill and Others, Transactions of Plasma Science, Vol. 21, No. 3,June 1993, 289-290.

An example of a non-transferable plasma arc generator with magnetic railacceleration is described in SU 890567. In that generator, theelectrodes are in form of two coaxial elliptical tubes and the spacebetween the electrodes holds a dielectric material. A wall of each ofthe tubes is axially slotted such that the slot in one tube faces anon-slotted wall portion of the other tube. Adjacent to each slot thereis one electric contact and in this way a two-rail structure isachieved. For uninterrupted circulation of the plasma arc it must becapable of crossing the slots and to this end the width of each slotmust be less than the thickness of the arc. However, when crossing anyof the slots the arc arrives exactly at the zone of the adjacentelectric contact, where direction of its further movement is indefinite,and consequently the speed at which the arc moves near the slots isreduced and the discharge is occasionally even interrupted, which is anobvious disadvantage.

SU 847533 describes a transferable plasma arc generator for treating anelectrically conductive substrate. It comprises a main electrode formingpart of the generator and the electrically conductive substrate is setas the counter electrode. The main electrode is in form of a spirallywound hollow longitudinal body having one winding whose partiallyoverlapping ends are angularly displaced relative each other to form agap between them. The rim of one end of the spiral body is placed inproximity of the substrate (proximal rim) and is connected to a pole ofan electric power supply by connector means being situated near saidgap. The spiral configuration of the electrode complies with thefollowing equation:

Y=K(X)^({fraction (3/2)})

where Y is the spiral pitch, K is a coefficient of proportionality and Xis the linear distance along the spiral's circumference between theconnector means and the spiral's end. Compliance with that equationallegedly ensures acceleration of the arc along the spiral electrode.

However, use of an electrode whose configuration meets the stipulationsof the above relationship is associated with a number of shortcomings:

(a) manufacture of the spiral electrode from graphite or tungsten orsome other material conventionally used for making electrodes for plasmaarc generators, is difficult and expensive;

(b) due to the exponential increase of Y as a function of X, the plasmacurrent fluctuates and consequently, in practice, a plasma arc generatoraccording to SU 847533 is capable of operating reliably withoutauxiliary means only up to a spiral diameter of not more than 6 cm.,while at larger diameters interruptions of the plasma arc might occur.To preempt such interruptions, the plasma arc discharge must bere-ignited at every cycle by means of a high-voltage oscillator;

(c) since the plasma is accelerated non-uniformly along the spiralproximal electrode rim, the electrode is heated in a non-uniform fashionwhich requires an efficient and reliable water cooling system withappropriate instrumentation for effective water temperature and pressurecontrol. All this renders the plasma generator expensive and rendersimpossible its applications for missions where use of cooling water isundesirable because of the dangerous consequences of any leakage.

OBJECTS OF THE INVENTION

It is one object of the present invention to provide a simple andinexpensive electrode for a plasma arc generator, adapted to generate acontinuously circulating, self-stabilized plasma arc with no need forany water cooling or injection of a protecting gas, and which at leastup to an output of about 50 kW may operate for considerable spans oftime.

It is another object of the invention to provide a plasma generatorincluding the novel electrode.

It is yet another object of the present invention to provide atransferable arc type plasma generator of the kind specified suitablefor heat treatment of solidifying liquid metal in molds.

It is a still further object of the present invention to provide animproved process for heat treatment of solidifying liquid metal in moldswith a circulating plasma arc.

GENERAL DESCRIPTION OF THE INVENTION

In the following description and claims the terms “longitudinal” and“longitudinally” are used in relation to a plasma arc generatingelectrode with a tubular body with two terminal rims, to describe anypath or direction along the wall of the tubular body that leads from onerim to the other; and the terms “lateral” and “laterally” signify adirection intersecting a longitudinal line.

By one of its aspects, the invention provides a plasma arc generatorelectrode which in association with a counter electrode provides atwo-rail structure capable of generating a plasma arc dischargedisplaceable along a closed path in a first direction, which electrodehas electric connector means for connection to a d.c. source of electricpower supply and comprises an essentially tubular body with a first rimforming part of a first rim region, and a second, working rim formingpart of a second rim region and serving for the electric arc discharge,in which electrode:

(i) said electric connector means include at least one connector site onthe electrode;

(ii) said tubular body has at least one longitudinally extending gapwith a first rim region gap stretch, a main gap stretch and a second rimregion gap stretch, each of which gaps divides laterally between twowall sectors each having first and second rim portions, one of said wallsectors carries a connector site associated with the gap;

(iii) the second rim portion of one of said wall sectors has a plasmaarc transmitting zone, and the second rim portion of the other wallsector carrying said connector site has a plasma arc receiving zone,which plasma arc transmitting and receiving zones are separated by andborder on the second rim region gap stretch of said longitudinallyextending gap, thus forming the two sides of said gap stretch;

(iv) said gap-associated connector site is so located that itsprojection on a second rim portion is laterally removed from said plasmaarc-receiving zone in a second direction being opposite to said firstdirection,

whereby in operation a Lorentz force is generated in said two-railstructure causing a plasma arc formed between said plasma arc generatorelectrode and counter electrode to move uninterruptedly in a closed pathin said first direction along said second rim region and across each ofsaid second rim region gap stretches.

The essentially tubular body of a plasma generator electrode accordingto the invention may be cylindrical, prismatic, polyhedral with astar-shaped profile and the like.

In accordance with one embodiment of the invention, said tubular bodyhas one single gap and said two wall sectors merge into a single bodyextending from one side of the gap to another. Thus, in accordance withthis embodiment the electrode has one single slotted tubular body.

In accordance with another embodiment of the invention, said tubularbody has several gaps and several wall sectors, each wall sectorextending between two gaps.

The portion of a plasma arc that is in contact with the second rimregion of the generator electrode is referred to in the art as “foot”.In operation of a plasma arc generator electrode according to theinvention the plasma arc foot moves in a closed path along the secondrim region.

In accordance with a preferred embodiment of a plasma arc generatorelectrode according to the invention, each second rim region gap stretchis so dimensioned as to be essentially not wider than the smallestdiameter of the actual plasma arc column; and the distance between saidprojection of the gap-associated connector site on to a second rimportion and said electric arc receiving zone is essentially not smallerthan the largest diameter of the foot of the actual plasma arc column.

It is noted that the diameter of the arc column and the diameter of thearc foot are visibly determinable values, which may be measuredexperimentally. Values of the smallest and largest arc column diametersmay moreover be calculated from values of the largest and the smallestarc currents, with the aid of equations known to persons skilled in theart. For example, in a gaseous environment at atmospheric pressure, andat an arc current of about 300 A the arc column diameter on a solidelectrode will reach about 5 cm, and the diameter of the arc foot isusually within the range of from 3 to 5 mm.

The meaning of the above provisions is that the narrowest possible arccolumn initiated in the device should be able to cross a gap, and thewidest foot of the arc should not overlap a zone underlying a connectorsite while crossing a second rim region gap stretch, but rather movethrough the electric arc receiving zone that is laterally removed fromthe connector site in the manner specified, whereby uninterruptedmovement of the electric arc is ensured.

Preferably the connector sites are placed in proximity to the first rimregion.

If desired, the second rim region of the electrode may be bevelledwhereby the surface for the electric discharge is increased and deviatesfrom normal to the axis of the tubular body, thereby enabling to controlorientation of the arc.

In accordance with one embodiment of a plasma arc generator electrodeaccording to the invention the main stretch of said at least onelongitudinally extending gap is so shaped that the projection of saidgap-associated connector site on a second rim portion is located in thatwall sector that holds the electric arc transmitting zone.

According to one embodiment of the invention, the sectors of saidtubular body are so designed that the projection of each gap-associatedconnector site on a second rim portion is located off said closed path,either within or outside the perimeter of said closed path.

If desired, the wall sectors of the plasma arc generator electrodeaccording to the invention may be so designed that at least the secondrim region stretch of each gap is formed by an overlap between adjacentwall sector portions comprising said plasma arc transferring andreceiving zones. In such a configuration, the cross-sectional area ofthe electrode is increased beyond a cylindrical tubular body whoseperimeter is defined by the connector sites on the first rim. Forexample, the tubular body of the electrode may have a star-likepolyhedral shape and be assembled from a plurality of modular bodysegments partially overlapping near their edges.

When powered, a plasma generator electrode according to the invention,e.g. of graphite or a refractory metal is capable of generating a plasmaarc discharge of up to 50 kW power, without the need for water cooling.However, for electrodes according to the invention with across-dimension not exceeding 7 cm, operation with interruptions may berequired.

According to a second aspect of the invention there is provided a plasmaarc generator apparatus comprising an electrode of the kind specified.The plasma arc generator apparatus may be of either the non-transferableor transferable type. A non-transferable plasma arc generator apparatusaccording to the invention may be utilized for the plasma treatment ofnon-conductive substrates such as raw materials for the buildingindustry, waste or any other dielectric material.

By one embodiment, the invention provides a transferable plasma arcgenerator apparatus comprising a plasma arc generator electrode forcooperation with an electricity conducting substrate serving as acounter electrode, which plasma arc generator electrode and counterelectrode form together a two-rail structure capable of generating aplasma arc discharge displaceable along a closed path in a firstdirection, which plasma arc generator electrode has electric connectormeans for connection to a d.c. source of electric power supply andcomprises an essentially tubular body with a first rim forming part of afirst rim region, and a second, working rim forming part of a second rimregion and serving for the electric arc discharge, in which electrode:

(i) said electric connector means include at least one connector site onthe electrode;

(ii) said tubular body has at least one longitudinally extending gapwith a first rim region gap stretch, a main gap stretch and a second rimregion gap stretch, each of which gaps divides laterally between twowall sectors each having first and second rim portions, one of said wallsectors carries a connector site associated with the gap;

(iii) the second rim portion of one of said wall sectors has a plasmaarc transmitting zone, and the second rim portion of the other wallsector carrying said connector site has a plasma arc receiving zone,which plasma arc transmitting and receiving zones are separated by andborder on the second rim region gap stretch of said longitudinallyextending gap, thus forming the two sides of said gap stretch;

(iv) said gap-associated connector site is so located that itsprojection on a second rim portion is laterally removed from said plasmaarc-receiving zone in a second direction being opposite to said firstdirection,

whereby in operation a Lorentz force is generated in said two-railstructure causing a plasma arc formed between said plasma arc generatorelectrode and counter electrode to move uninterruptedly in a closed pathin said first direction along said second rim region and across each ofsaid second rim region gap stretches.

In the following description a plasma arc generator electrode accordingto the invention forming part of a plasma arc generator apparatus willbe referred to occasionally as “main electrode”.

In one embodiment, the transferable plasma arc generator apparatusaccording to the invention comprises a cylindrical housing surroundingthe main electrode and spaced therefrom so as to form with it an annularchamber. If desired, a lid may be provided for sealing the housing fromthe end proximal to the electrode's first rim. Further if desired,ignition means for igniting a plasma arc discharge may be mounted withinthe annular space between the housing and the main electrode inproximity of the first rim, whereby upon ignition an auxiliary arc isgenerated which initiates the main arc.

Typically the ignition means may comprise a first stem-like electrodeheld within a second, coaxial tubular electrode in a spacedrelationship, which first and second electrodes are connectable to thetwo poles of the d.c. electric power supply, a third, rod-shapedelectrode being mounted substantially normal to said second tubularelectrode at an end portion thereof, which third electrode iselectrically connectable to a high voltage oscillator. Preferably, saidend portion of the tube is formed with an inner ledge so as to define anarrowed gap between the stem-shaped and tubular electrodes in theregion where the high oscillation voltage is applied via the third,rod-shaped electrode.

By one particular design, the ignition means is secured to the lid ofthe housing and extends axially to the region of the second rim of themain electrode.

According to a preferred embodiment of the transferable plasma arcgenerator apparatus according to the invention, means are provided foraxial displacement of the main electrode whereby the distance of thesecond rim from the substrate may be adjusted and optimized in thecourse of operation.

A typical application of a transferable plasma arc generator apparatusaccording to the invention is the heat treatment of a liquid metalduring solidification in a suitable mold such as an ingot mold.

Accordingly, by yet another aspect the invention provides a process ofheat treatment of a solidifying liquid metal inside a mold, comprisingproviding a transferable plasma arc generator apparatus having a mainelectrode for cooperation with an electricity conducting substrateserving as a counter electrode, which main electrode in association withsaid electricity conducting substrate provides a two-rail structurecapable of generating a plasma arc discharge displaceable along a closedpath in a first direction, which main electrode has electric connectormeans for connection to a d.c. source of electric power supply andcomprises an essentially tubular body with a first rim forming part of afirst rim region, and a second, working rim forming part of a second rimregion and serving for the electric arc discharge, in which electrode:

(i) said electric connector means include at least one connector site onthe electrode;

(ii) said tubular body has at least one longitudinally extending gapwith a first rim region gap stretch, a main gap stretch and a second rimregion gap stretch, each of which gaps divides laterally between twowall sectors each having first and second rim portions, one of said wallsectors carries a connector site associated with the gap;

(iii) the second rim portion of one of said wall sectors has a plasmaarc transmitting zone, and the second rim portion of the other wallsector carrying said connector site has a plasma arc receiving zone,which plasma arc transmitting and receiving zones are separated by andborder on the second rim region gap stretch of said longitudinallyextending gap, thus forming the two sides of said gap stretch;

(iv) said gap-associated connector site is so located that itsprojection on a second rim portion is laterally removed from said plasmaarc-receiving zone in a second direction being opposite to said firstdirection,

installing said plasma generator so that said second rim is proximal tothe surface of the liquid metal at a suitably selected distancetherefrom, connecting said main electrode to one pole of an electricpower supply and the liquid metal to the other pole thereof, igniting anelectric arc, whereby in operation a Lorentz force is generated in atwo-rail structure comprising said main electrode and said counterelectrode, causing a plasma arc formed between said main electrode andcounter electrode to move uninterruptedly in a closed path in said firstdirection along said second rim region and across each of said secondrim region gap stretches;

and continuing the treatment until the liquid metal reachessolidification.

The control of the chilling and solidifying regime of a liquid metal byheat treatment with a plasma arc in accordance with the invention,improves the quality of the solidified metal. In accordance with theinvention it was found that such improvement is due to the displacementof the plasma arc along a closed path by action of a Lorentz forcegenerated inside the novel plasma generator. It has further been foundin accordance with the present invention that due to such treatment,prior art casting defects such as formation of blowholes and porosity,segregation, formation of contraction cavities and inhomogeneity ofchemical composition and crystal structure across the ingot, areavoided. It has also been found that in accordance with the inventionthe amount of waste metal is reduced. Still further it has been foundthat, as a consequence of the heat treatment according to the inventionthe crystalline structure of the solidified metal is improved, possiblyin consequence of the electromagnetic fields which account for thecreation of the Lorentz force.

BRIEF DESCRIPTION OF THE DRAWINGS

For better understanding, some specific embodiments of the inventionwill now be described, by way of example only, with reference to theannexed drawings in which:

FIG. 1 is a schematic three-dimensional view of one embodiment of aplasma arc generator electrode according to the invention;

FIG. 2A is a side view of another embodiment of an electrode accordingto the invention, also showing schematically a counter-electrode;

FIG. 2B is a top view of the embodiment shown in FIG. 2A;

FIG. 3 is a schematic three-dimensional view of yet another embodimentof a plasma arc generator electrode according to the invention, togetherwith a counter-electrode;

FIG. 4 is a schematic three-dimensional view of yet another embodimentof a plasma arc generator electrode according to the invention;

FIG. 5 is a schematic cross-sectional view of one embodiment of anon-transferable plasma arc generator apparatus according to theinvention;

FIG. 6 is a schematic cross-sectional view of one embodiment of atransferable plasma arc generator apparatus according to the invention;

FIG. 7A is a schematic axial cross-sectional view of another embodimentof the transferable plasma arc generator apparatus according to theinvention;

FIG. 7B is a bottom view of the embodiment shown in FIG. 7A;

FIG. 8 is an enlarged cross-sectional view of ignition means in a plasmaarc generator apparatus according to the invention;

FIG. 9 is a general view of a setup for the implementation of controlledchilling and solidification of liquid metal in a mold, by means of aplasma arc generator apparatus according to the invention; and

FIG. 10 shows ingots solidified with and without treatment by acirculating plasma arc according to the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 illustrates a perspective view of one embodiment of a plasma arcgenerating electrode according to the invention. As shown, electrode 2comprises a tubular cylindrical body having a longitudinal axis, a firstrim 3, a second, working rim 4 serving for the electric arc dischargeand being a constituent of a two-rail structure which in operationdefines a closed path for the movement of the electric arc inconsequence of a Lorentz force generated in the device. Side wall 5 ofthe cylindrical electrode body is sliced by a single throughgoing gap 6generally extending in the axial direction and having a first rim regiongap stretch 7, a main gap stretch 8 and a second rim region gap stretch9. As shown, the main gap stretch 8 comprises two parts forming betweenthem an obtuse angle. Gap 6 divides between two sectors 10 and 11 ofwall 5. Electrode 2 has on the first rim 3 a gap-associated connectorsite 12 fitted with a connector 13 serving for connection to a pole of ad.c. power source (not shown). It is noted, however, that the connectorsite need not necessarily be located on the first rim and may bepositioned at any level of the tubular body, but preferably at areasonable distance from the working rim 4 so as not to be affected bythe plasma arc and substrate fumes. The dashed arrow 14 in FIG. 1 showsthe direction of movement of the generated electric arc in operation inconsequence of the Lorentz force, i.e. the so-called first direction. Asmentioned, for the purpose of this movement, the electrode 2 with thesecond rim 4 is one component of the required two-rail structure and thecounter electrode 15 constitutes the other component.

The second rim region gap stretch 9 divides between an electric arctransmitting zone 16 and an electric arc receiving zone 17. Thereceiving zone 17 is on the same wall sector 11 as the connector site12.

As is seen, in this embodiment, gap 6 is so shaped that projection 19 ofthe connector site 12 on the second rim 4 of the electrode 2 is locatedclose to the electric arc transmitting zone 16 and is removed from thearc receiving zone 17 in a direction (the so-called second direction)that is opposite to the mentioned first direction by a distance L. Thisdistance is essentially not smaller than the largest diameter of thefoot of the generated plasma arc column.

When the arc is initiated between the electrode 2 and the counterelectrode 15, it forms a current conducting plasma body bridging the twoelectrodes. As the two electrodes constitute a two-rail structure, theelectric current creates a magnetic field which interacts with thecurrent of the arc and its magnetic field, thus causing the generationof the Lorentz force which drives the arc column along the second rim 4in the direction away from the projection 19 of the connector site 12,i.e. in the direction indicated by the dashed arrow 14.

According to the invention, the uninterrupted movement of the plasma arcis achieved due to the fact that on each crossing of the second rim gapstretch 9 the plasma arc foot is downstream (with reference to themovement of the arc in the direction of arrow 14) a zone of electricalinfluence of the connector site 12, i.e. downstream of projection 19.

FIGS. 2A and 2B illustrate another embodiment of an electrode accordingto the invention, comprising a rectangular tubular body 20 assembledfrom a number of segments forming the electrode wall sectors 21 andseparated by a plurality of slanted gaps 22. The upper edges of thesegments 21 form a first rim 24 of the electrode 20, and the lower edgesthereof form a second rim 27 thereof, each of sectors 21 thus havingfirst and second rim portions. Each of the electrode sectors 21 isprovided with an electric connector site fitted with laterallyprojecting connectors 23 and positioned at the upper inner portion ofthe sectors 21 close to the first rim thereof. All connectors 23 areinterconnected by a common current carrying plate 25 electricallyconnectable to a pole of a d.c. power source (not shown) via a currentcarrying bus 26. Essentially the location of each gap-associatedconnector 23 relative to the associated gap 22 and of the electric arctransmitting and receiving zones on the two sides of the second rimregion gap stretch, as well as the location of the projection of eachconnector site on a second rim portion are all similar to thearrangement shown in FIG. 1, though the shapes and numbers of thesectors and gaps are different. As can be seen, the projection of eachconnector 23 associated with a particular electrode body sector 21, to aplane holding the second rim 27 of the electrode 20 falls on to theadjacent electrode segment, close to its plasma arc transferring zone.In FIGS. 2A and 2B there is schematically shown a counter electrode 28positioned under the second rim 27 of the electrode 20. The counterelectrode is provided with a terminal 29 for connection to the oppositepole of the d.c. power source (not shown). When an electric arcdischarge is initiated between electrodes 20 and 28, a Lorentz force isgenerated by which the plasma arc is displaced uninterruptedly along thesecond working rim 27 of the tubular body in the direction of a dottedarrow in FIG. 2B (first direction).

FIG. 3 illustrates yet another embodiment of an electrode 30 accordingto the invention, having a star-like shape and comprising an essentiallytubular body assembled from a plurality of frusto-triangular segmentsforming a plurality of wall sectors 31 separated by axially extendinggaps 32. In the axial direction the tubular body of the electrode 30extends between a first (upper) rim 33 and a second (lower), working rim34. The frusto-triangular wall sectors 31 have each a first wall portion35 which holds the plasma arc receiving zone and also an electricconnector 37, and a second wall portion 36 which holds the plasma arctransmitting zone. The edge 38 of a first portion 35 of a sector 31 thatis close to an associated gap 32 is referred to herein as a proximaledge, and the opposite edge 39 of a second portion 36 of an adjacentsector 31 is referred to herein as distal edge 39. The electricconnector means 37 of all the electrode sectors 31 are connected to acommon current carrying plate 40 provided with a bus 41 for connectingto a pole of a d.c. power source (not shown). Underneath the electrode30 there is shown schematically a counter electrode 42 with a terminal43 for connection to the opposite pole of the d.c. power source (notshown).

It can be seen that the electrode sectors 31 are arranged in such amanner, that projections of the connectors 37 on the second rim 34 aresituated within the perimeter of the closed path of the arc movement insaid first direction, shown by way of the dashed arrow. Moreover, eachfirst portion 35 of a sector 31 partially overlaps the second wallportion 36 of an adjacent electrode sector 31 with the formation of saidgaps 32. Thus, each proximal edge 38 with the associated connector 37 isremoved from the adjacent distal edge 39 in a second direction beingopposite to said first direction, by a distance L. In this specificembodiment this clearance is also the distance between the electric arcreceiving zone and the projection of the site of the electric connectormeans 37 on the second rim 34. (As defined, the arc transmitting zoneand the arc receiving zone form sides of each of the gaps 32 at thesecond rim's 34 region.) Owing to that arrangement, each electric arctransmitting zone (not seen) transmits the moving arc column to theadjacent arc receiving zone across the second rim region gap stretch ata location which is downstream from the site of the connector 37, thusensuring the uninterrupted movement of the arc in the said firstdirection of the dashed arrow.

FIG. 4 shows schematically yet another embodiment 44 of an electrodeaccording to the invention. Similar as in the embodiment of FIG. 3, inthat the gaps are axial with their first rim region gap stretch, maingap stretch and second rim region gap stretch being aligned, and also inthat the projections of the connector means 45 on to a plane P holdingthe second working rim 46 of the electrode 44, are off the closed path47 of the plasma arc movement on the same plane P. However, as distinctfrom the embodiment of FIG. 3, the projections of the connector means 45fall outside the perimeter of the path 47, and the wall sectors 48 donot overlap one another near gaps 49. Similarly as in FIG. 3, eachprojection of a connector 45 on plane P holding the second rim 46 isremoved from an associated plasma arc transmitting zone in a directionopposite to that of the movement of the plasma arc, by a distance Lwhereby in operation uninterrupted movement of the plasma arc along itsclosed path is ensured.

All the electrode embodiments illustrated in FIGS. 1 to 4 are designedfor providing an uninterrupted circulating plasma arc discharge inplasma generators. As mentioned, the width of the second rim region gapstretch should preferably be not greater than the diameter of thenarrowest arc column designed to be initiated on the electrode, and thedistance L should preferably be not smaller than the widest foot of anarc generated on the electrode. The inventive configuration of theelectrode allows to use it for relatively large electrodes without anywater cooling and injection of a protecting gas for stabilizing theplasma discharge, and at least up to power output of about 50 kW.

FIGS. 5 and 6 illustrate schematically and by way of example only,embodiments of plasma generator apparatus according to the invention of,respectively the non-transferable and transferable types.

Referring first to FIG. 5, there is shown in an axial cross-sectionalview one embodiment of a plasma generator apparatus 50 comprising a maintubular electrode 51 according to the invention having a slantingthroughgoing gap 52 and being provided with electric connector means 53.The main electrode 51 is concentrically surrounded by a conductivecylindrical housing 54 having a lid 55. It is noted that lid 55 isoptional. The main electrode 51 and the housing 54 are connected to twoopposite poles of a high current d.c. power source 56, as known per se,with the housing 54 serving as the counter electrode in the apparatus.The apparatus 50 is also provided with ignition means 57 for initiatingan auxiliary arc discharge. The ignition means comprise an ignitionelectrode 58 energized from a high voltage oscillator 59 as known perse, and a protrusion 60 provided on the inner wall of the housing andpositioned close to the main electrode 51 serves to facilitate ignitionof an auxiliary arc 61 which upon ignition moves to the lower rim regionof the main electrode. The vertical displacement of the auxiliary arc isalso caused by the Lorentz force, which in this particular case appearsowing to existence of a current carrying, rail-like structure comprisingthe main electrode 51 and the housing 54. The main arc discharge 62 isestablished between the lower rim region of the main electrode 51 andthe counter electrode 54, and starts to circulate around the lower rim63 of the tubular electrode 51, thus providing heat treatment of asubstrate 64 (for example, a concrete slab).

FIG. 6 illustrates schematically a cross-sectional view of atransferable plasma arc generator apparatus 70 according to theinvention. A main tubular electrode 71 of the apparatus has theabove-described configuration and is connected to a positive pole of thed.c. power source 72, the opposite, negative pole being connected to anelectrically conductive substrate 73 which is the object to be treatedand serves as counter electrode. The negative pole of the power source72 is also connected to a cylindrical housing 74 concentricallysurrounding the main electrode 71. The lower portion of the inner wallof the housing 74 is covered by a high-temperature resistant,electrically insulating layer, for example, painted by a suitable paint(not shown). An ignition electrode 75 is mounted in the annular spaceformed between the main electrode and the housing. When the ignitionelectrode 75 is energized by a high voltage oscillator 76, an auxiliaryarc 77 is generated between the main electrode and the ignitionelectrode, and is then transferred downwards to the lower rim region 78of the main electrode 71. The lower rim 78 region is bevelled in amanner shown in the drawing, thus providing the desired shape andorientation of the main arc discharge 79. The bevelled rim region 78 andthe painted wall of the housing 74 cause the arc 79 to span from the rim78 to the surface 73, rather then to the housing 74.

FIGS. 7A and 7B show schematically an axial cross-sectional view andbottom view, respectively, of yet another embodiment 80 of atransferable plasma generator apparatus according to the invention. Theapparatus comprises a main tubular electrode 81 mounted within acylindrical housing 82 sealed from above by a cover 83, which latter isoptional. The generator is connected to a d.c. power supply unit 84including a high current source and a high voltage oscillator (notshown) serving for energizing the main and counter electrodes and theignition means 85 of the apparatus. The longitudinal axis of the mainelectrode 81 is vertical to the surface of an object to be treated,e.g., a metal piece, which is set as a counter electrode 86. The housing82 that accommodates the main electrode 81, is installed at a distance Wfrom the surface of the metal piece to provide for a working space for aplasma arc discharge. The main electrode 81 according to the invention,may be manufactured from graphite or from electrically conductive,erosion resistant refractory material. The ignition means 85 protrudesfrom the cover 83 and is situated in the annular space formed betweenthe main electrode 81 and the housing 82. An electrically conductiveconnector 93 is releasably mounted in the cover 83 and is electricallyconnected at one end to the power supply unit 84, and at its oppositeend to the main electrode 81 so as to supply electrical power thereto.

A gap 88 shown in FIG. 7A extends from the first (top) rim 89 of thecylindrical tubular main electrode 81 down to the second (bottom),working rim 90 thereof, and has a first rim region gap stretch 91, amain gap stretch and a second rim region stretch 92. As further shown inFIG. 7A, the gap 88 comprises two parts, a vertical one which isparallel to the generatrix of the cylindrical side wall of the electrode81, and a slanting one, which parts include between them an obtuseangle. Due to this design of gap 88, the first and second rim region gapstretches 91 and 92 are not in alignment and are angularly displaced asshown in FIG. 7B. The electrode 81 comprises one electrode sector fittedwith one electric connector 93 mounted in a lid 83 by means of aninsulating sleeve and having its site at the first rim 89 of theelectrode in close proximity to the first rim region gap stretch 91. Theprojection of the connector 93 on to the second rim 90 is locatedbetween the second rim region gap stretch 92 and the projection of thefirst rim region gap stretch 91 on to second rim 90, at a distance Lfrom stretch 92 in a direction opposite to that of the movement of theplasma arc shown by the arrows in the circular dashed line 94.

FIG. 8 illustrates one embodiment of the ignition means in a plasma arcgenerator apparatus according to the invention, e.g. that shown in FIG.7A under reference number 85. The ignition means 85 may be releasablyfitted in the cover 83 of the apparatus of FIGS. 7A and 7B so as toproject between the main electrode 81 and the sidewall of the housing82. However, other locations of the ignition means are conceivable. Inthe embodiment shown in FIG. 8, the ignition means 85 consists of afirst, second and third electrodes 95, 96 and 97 which are electricallyconnected to the power unit 84 and secured within a high voltageinsulating cap 98. The electrode 95 is in form of an elongated stempartially and coaxially accommodated within the second, tubularelectrode 96 in a spaced relationship with the formation of an annularspace 99. The third electrode is in form of a horizontal rod 97 mountednear the upper edge of the tubular electrode 96 with the inner end closeto electrode 95. The electrode 97 is essentially normal to theelectrodes 95 and 96 and is electrically connected to the high voltageoscillator (not shown).

It is advantageous if the upper region of the tube 96 is formed with aninner ledge 100 so as to define a dedicated narrow gap betweenelectrodes 95 and 96 in the region where the high oscillation voltage isapplied.

Preferably the ignition means 85 are mounted remote from the workingspace W since in this way functioning thereof is not significantlyinfluenced by the hot and highly erosive atmosphere present in theworking space. In practice, it is recommended that the ignition means beformed as a module so as to enable fast and convenient maintenance andreplacement thereof.

The plasma arc generator apparatus illustrated in FIGS. 7A, 7B and 8 isput into effect in the following way. The power is switched on and aworking voltage of approximately 170 V is applied simultaneously withinthe working space between the main electrode 81 and the metal surface86, between the main electrode 81 and housing 82, as well as within theannular space 99 between the electrodes 95 and 96 of the ignition means85. Thereafter the high voltage oscillator is switched on so as tosupply oscillating high voltage sufficient for generating an electricaldischarge between electrode 97 and the ledge 100 and also a dischargebetween the ledge 100 and electrode 95. This arc discharge is followedby the formation of an auxiliary plasma arc within a gap between thecoaxially disposed electrode means 95 and 96. The plasma arc is shifteddownwards along the side wall of the main electrode 81 by virtue of railacceleration provided between respective parallel surfaces of thecylindrical housing 82 and the main electrode 81, and is pushed towardsthe second rim 90 of the main electrode 81 at a speed of about 40 m/sec.The full time required for the ignition step does not exceed 0.002 sec.After the auxiliary plasma arc generated by the ignition discharge hasreached the second rim 90, it acquires the shape of the main plasma arcdischarge 101 between the second rim 90 of the main electrode and thesurface 86 of the metal to be treated, which main plasma arc rotates inthe working space W.

FIG. 9 shows schematically how a plasma generator according to thepresent invention, can be used for heat treatment of a liquid metalsolidifying within an ingot mold.

The setup shown in FIG. 9 includes an ingot mold 120, which has a bottompouring arrangement with a pouring gate 121. The liquid metal 122 ispoured from a ladle (not shown) into a funnel 124 of the pouring gatesystem 121, enters the ingot mold 120 through the bottom thereof andfills it up to the height controlled by a sensor 125. Adjacent to theupper part of the mold 120, there is disposed a plasma arc generatorapparatus 126 containing a main electrode 127 according to the inventionheld in a carriage 128 having wheels 135 mounted on rails 129 and thuscapable of being reversibly shifted between a rest position out ofalignment with mold 120 and an operational position in alignment withthe mold. There are further provided means (not shown) capable oflifting and lowering the apparatus 126. The plasma arc generatorapparatus 126 comprises a main power source 130, a high voltageoscillator 131 and a control panel 132 for controlling the shifting ofthe apparatus 126 to and from the working position as well as itsfunctioning during the working cycle. To this end, control panel 132 isequipped with appropriate electronic control means (not shown) enablingoperation in a manual mode or in accordance with a preprogrammedschedule.

A bus 133 with appropriate electric cables is provided for electriccommunication between the power sources 130, 131 via the control panel132, with the plasma generator 126, the liquid metal 122 via a connector134, the mechanism 135 and the sensor 125.

In practice, the plasma generator 126 is brought into the workingposition above the ingot mold 120, the liquid metal is poured into themold up to a certain level controlled by the sensor 125, which leveldefines the width W of the working space between the surface of theliquid metal 122 in the mold and the second (bottom) rim of the mainelectrode 127. The width W is usually kept within the range of 8 to 10mm, if the operating voltage is within the range of 60-80 V. Foroperating voltages higher than 80 V the width is increased and at 170 V,for example, it is 25 mm. After the required width of the working spaceis adjusted, the power source 130 and the high voltage oscillator 131are switched on, whereby the auxiliary arc discharge is ignited andmaintained until the main plasma arc discharge is initiated and the heattreatment of the metal surface begins. The high voltage oscillator isusually kept on until establishment of the main arc discharge, which isindicated by an electrical current flow corresponding to the power,required for a particular application. For example, at a voltage 170 V amain arc discharge can be achieved with a current of 300 A, whichprovides for 50 kW of electric power. The height of the main electrode127 is approximately 40-60 mm for an ingot having the mass of about 20kg.

The duration of the main arc discharge, i.e. the time required for theheat treatment can be controlled by means of an appropriate timer (notshown). In practice the timer should be suitable for the continuous orperiodical actuation of the power source during solidification of theingot within a mold.

After termination of the heat treatment the plasma arc generatorapparatus is switched off and is shifted out of the working position,and upon further cooling the chilled ingot can be released from themold.

It should be noted, that owing to the steady circulation of the main arcdischarge achieved in accordance with the present invention, it ispossible to perform the required heat treatment while varying the widthof the working space. Thus, if desired, the plasma generator may beprovided with means (not shown) for vertically reciprocating the mainelectrode 127 within the housing 126, thereby adjusting the width ofworking space W (FIG. 7A). Such a vertical shift may be continuouslycontrolled by the sensor 125 monitoring the level of the liquid metal inthe mold, thus ensuring lowering of the electrode 127 in accordance withthe metal shrinkage, whereby the treatment which leads to theelimination of defects in the ingots is improved and the amount of wastemetal is reduced.

The result of a heat treatment according to the invention is illustratedin FIG. 10, which shows photographs of two ingots (a) and (b) fromaluminum alloy A332.0 solidified without (a) and with (b) treatment bythe circulating plasma arc technique according to the invention. Themass of the ingots is 7.2 kg. The conventional ingot (a) has a blowholein its upper portion, and consequently a significant layer of the ingotmust be cut away by the user. In contrast, the ingot (b), which wassubjected during chilling to plasma arc treatment according to theinvention for a period of 50 sec, has a smooth upper surface and doesnot require any additional treatment since it has the required precisedimensions.

We claim:
 1. A plasma arc generator electrode (2, 20, 30, 44) which inassociation with a counter electrode (15, 28, 42, 54, 73, 86, 122)provides a two-rail structure capable of generating a plasma arcdischarge displaceable along a closed path in a first direction (14),said plasma arc generator electrode having an electric connector means(13, 23, 37, 45, 53, 93) for connection to a d.c. source of electricpower supply (56, 72, 84) and comprises an essentially tubular body witha first rim (3, 24, 33, 89) forming part of a first rim region, and asecond, working rim (4, 27, 34, 46, 63, 78, 90) forming part of a secondrim region and serving for the electric arc discharge, in whichelectrode: (i) said electric connector means include at least oneconnector site (12) on the plasma arc generator electrode; (ii) saidtubular body has at least one longitudinally extending gap (6, 22, 32,49, 52, 88) with a first rim region gap stretch (7, 91), a main gapstretch (8) and a second rim region gap stretch (9, 92), each of whichgaps divides laterally between two wall sectors (10 and 11; 21 and 21;31 and 31; 48 and 48), each having first and second rim portions, one ofsaid wall sectors (11, 21, 31 48) carries a connector site associatedwith the gap; (iii) the second rim portion of one of said wall sectorshas a plasma arc transmitting zone (16, 36) and the second rim portionof the other wall sector carrying said connector site has a plasma arcreceiving zone (17, 35), which plasma arc transmitting and receivingzones are separated by and border on the second rim region gap stretchof said longitudinally extending gap, thus forming the two sides of saidgap stretch; (iv) said gap-associated connector site is so located thatits projection on a second rim portion is laterally removed from saidplasma arc receiving zone in a second direction being opposite to saidfirst direction, whereby in operation a Lorentz force is generated insaid two-rail structure causing a plasma arc formed between said plasmaarc generator electrode and counter electrode to move uninterruptedly ina closed path in said first direction along said second rim region andacross each of said second rim region gap stretches.
 2. The electrodeaccording to claim 1, wherein each second rim region gap stretch (9, 92)is so dimensioned as to be essentially not wider than the smallestdiameter of an actual plasma arc column; and the distance (L) betweensaid projection of the gap-associated connector site on to a second rimportion and said electric arc receiving zone is essentially not smallerthan the largest diameter of the foot of the actual plasma arc column.3. The electrode according to claim 1, wherein said tubular body of theplasma arc electrode (2, 51, 71, 81) has one single gap (6, 52, 88) andsaid two wall sectors merge into a single body extending from one sideof the gap to another.
 4. The electrode according to claim 1, whereinsaid tubular body has several gaps (22, 32, 49) and several wall sectors(21, 31, 48), each wall sector extending between two gaps.
 5. Theelectrode according to claim 1, wherein in said at least onelongitudinally extending gap (6, 22, 52, 88), the said first and secondrim region gap stretches (7 and 9, 91 and 92) are non-aligned.
 6. Theelectrode according to claim 5, wherein said main gap stretch (8, 52,88) has two parts including between them an obtuse angle.
 7. Theelectrode according to claim 5, wherein said at least one longitudinallyextending gap (22) is slanted.
 8. The electrode according to claim 1,wherein each gap-associated connector site is at or in proximity of thefirst rim (3, 24, 33, 89) region.
 9. The electrode according to claim 1,wherein said second rim (4, 27, 34, 46, 63, 78, 90) region is bevelled.10. The electrode according to claim 1, wherein the main stretch of saidat least one longitudinally extending gap (6, 22, 52, 88) is so shapedthat the projection of said gap-associated connector site on a secondrim portion is located in that wall sector that holds the electric arctransmitting zone (16, 87).
 11. The electrode according to claim 1,wherein the sectors (31, 48) of said essentially tubular body are sodesigned that the projection of each gap-associated connector site on asecond rim portion is located off said closed path.
 12. The electrodeaccording to claim 11, wherein the sectors (31) of said essentiallytubular body are so designed that the projection of each gap-associatedconnector site on a second rim portion is located within the perimeterof said closed path.
 13. The electrode according to claim 11, whereinthe sectors (48) of said essentially tubular body are so designed thatthe projection of each gap-associated connector site on a second rimportion is located outside the perimeter of said closed path.
 14. Theelectrode of claim 1, wherein the wall sectors (31) of the plasma arcgenerator electrode according to the invention are so designed that atleast the second rim region stretch of each gap is formed by an overlapbetween adjacent wall sector portions comprising said plasma arctransferring (36) and receiving (35) zones.
 15. The electrode accordingto claim 1, wherein said tubular body (30) has a star-like polyhedralshape and is assembled from a plurality of modular frusto-triangularsegments (31) each constituting a wall sector and partially overlappingnear the gaps.
 16. A plasma arc generator apparatus (50, 70, 80, 126)comprising the plasma arc generator electrode according to claim
 1. 17.The plasma arc generator apparatus (70, 80, 126) according to claim 16,wherein said plasma arc generator electrode (71, 81, 127) is capable ofcooperating with an electricity conducting substrate (73, 86, 122)serving as the counter electrode and forming together with said plasmaarc generator electrode the two-rail structure.
 18. The apparatus ofclaim 17, comprising a cylindrical housing (74, 82) surrounding the saidplasma arc generator electrode and spaced therefrom so as to form withit an annular chamber.
 19. The apparatus of claim 18, comprising a lid(83) sealing the housing from the end proximal to the first rim of theelectrode.
 20. The apparatus of claim 18, comprising ignition means (75,85) mounted within an annular space between said electrode and housing.21. The apparatus of claim 20, wherein said ignition means are mountedin proximity of said first rim.
 22. The apparatus of claim 1, comprisingmeans (132) for axial displacement of the plasma arc generatingelectrode.
 23. A process of heat treatment of a solidifing liquid metalinside a mold, comprising providing a transferable plasma arc generatorapparatus (70, 80, 126) having a main electrode (2, 20, 30, 44, 71, 81,127) for cooperation with an electricity conducting substrate (73, 86,122) serving as a counter electrode, which main electrode in associationwith said electricity conducting substrate provides a two-rail structurecapable of generating a plasma arc discharge displaceable along a closedpath in a first direction (14), which main electrode has electricconnector means (13, 23, 37, 45, 93) for connection to a d.c. source ofelectric power supply (56, 72, 84, 130) and comprises an essentiallytubular body with a first rim (3, 24, 33, 89) forming part of a firstrim region, and a second, working rim (4, 27, 34, 46, 78, 90) formingpart of a second rim region and serving for the electric arc discharge,in said main electrode: (i) said electric connector means include atleast one connector site (12) on the electrode; (ii) said tubular bodyhas at least one longitudinally extending gap (6, 22, 32, 49, 88) with afirst rim region gap stretch (7, 91), a main gap stretch (8) and asecond rim region gap stretch (9, 92), each of which gaps divideslaterally between two wall sectors (10 and 11; 21 and 21; 31 and 31; 48and 48) each having first and second rim portions, one of said wallsectors (11, 21, 31, 48) carries a connector site associated with thegap; (iii) the second rim portion of one of said wall sectors has aplasma arc transmitting zone (16, 36), and the second rim portion of theother wall sector carrying said connector site has a plasma arcreceiving zone (17, 35), which plasma arc transmitting and receivingzones are separated by and border on the second rim region gap stretchof said longitudinally extending gap, thus forming the two sides of saidgap stretch; (iv) said gap-associated connector site is so located thatits projection on a second rim portion is laterally removed from saidplasma arc receiving zone in a second direction being opposite to saidfirst direction, installing said plasma generator so that said secondrim is proximal to the surface of the liquid metal (122) at a suitablyselected distance therefrom, connecting said main electrode to one poleof the electric power supply (130) and the liquid metal to the otherpole thereof, igniting an electric arc, whereby in operation a Lorentzforce is generated in a two-rail structure comprising said mainelectrode and said counter electrode, causing a plasma arc formedbetween said main electrode and counter electrode to moveuninterruptedly in a closed path in said first direction along saidsecond rim region and across each of said second rim region gapstretches; and continuing the treatment until the liquid metal reachessolidification.
 24. The process of claim 23, comprising lowering saidplasma arc generating electrode (127) so as to maintain a constantdistance between said second rim and the surface of the metal (122)inside the mold.